Leap Pads and AR Coatings

[Pete Hanlin]

This may actually be a question for Steve (who has a lot of knowledge concerning thin coatings). Once again, Pete has been working in the lab and has noticed something puzzling...

When I edge an AR coated job (we use mainly Pentax, but I've noticed this effect with most every ARC we use), I will often notice a residual "imprint" of the leap pad on the surface of the lens. Most of the time, this imprint can be immediately wiped clean. However, when you steam up the lens, there is the imprint again.

I don't think it is caused by pressure, as I would then expect to find an imprint shaped like the chuck. Oddly enough, I've cleaned glasses that are months old that still show evidence of the leap pad when steamed up.

What is the leap pad doing to the ARC? Is it pulling something off the lens when it is peeled off? Is it transferring something to the lens? On more expensive lenses (1.66, e.g.) I use those little blue surface saver dots. Should I be using them for all ARC lenses? BTW, I notice they leave somewhat of an imprint as well.

Pete

[Steve Machol]

Good question Pete!

I honestly don't know the answer to this. One of two things is happening.

1. Residue from the leap pad glue is altering the chemistry of the AR surface.

2. A small portion of the AR surface is being removed along with the leap pad.

My suspicion is that the first scenario is the most likely one, however it's entirely likely that it's a combination of both facors.

Back in the days when I was someone important ( ;) ) and had tons of equipment at my disposal, I could've analyzed these surfaces with an Atomic Force Microsope (AFM) to see if the surface structure had been altered. Unforutunately my toys were taken away!

[JRS]

Pete,
I had similar experiences some years back when we blocked (using leap pads) and left them on the lens for very long. It appeared to be, at least partly, a chemical reaction. We quickly discovered that the liklihood of this was decreased, if we blocked then edged right away. And immediately removed the pad. We also discovered that the "quality" of the leap pad was also part of the chain - cheaper not always equals better. Better quality pads seemed to exibit less of the marking.
However, I would not rule out pressure completely. The typical pressure exerted by the chuck is in the neighborhood of 125 to 150psi - depending on the edger. With increasing 'thin' centered AR lenses, the amount of flexing that occurs is increased when the ram (air chuck) contacts and compresses the lens to the block surface. You don't mention what type of blocks you use - metal or some form of plastic, but look for how contact is made. Pad contact in a small, central area, will increase flex, while contact at the outer edges of the block can cause crazing. If you think the pressure might be part of the issue, I can email you a 1-page document on procedures for edging thin-centered lenses (Essilor document).

John

[Steve Machol]

Pressure and time undoubtedly play a significant role. The higher the pressure and longer the pad is on the lens, the greater the adhesion between pad and surface. And the greater the adhesion, the more likely that chemical reaction and/or coating loss (by removing the pad) could result.

[Joann Raytar]

Pete,

I have noticed the same thing, mostly on Pentax Surpass and sometimes on the Seiko 1.67 AR's. On the Pentax not only the leap pads but our chuck pads have left rings. I haven't noticed "Ring around the OC" on Crizal or Zeiss coatings yet.

How hard, on average, is the coating on these lenses? Or is their toughness more an issue of the thickness of the coating?

[double vision]

I am reasonably sure it's the amount of time the pad is in contact with the AR'd lens. I have tried many things including blue chips and clamp pressure, but it always came down to the amount of time. It doesn't matter what brand, I've seen it happen to all of them.

[Pete Hanlin]

I am inclined to agree to the time theory... I took an AR coated blank, applied a leap pad, and just let it sit unchucked for an hour. When I removed the leap pad, there was the impression!

So, we're back to either the leap pad transferring something to the lens or the leap pad removing part of the AR coat. My guess is that the pad is removing the hydrophobic coating from the lens...

Pete

[bwheeler]

I read in a trade article that allowing the lenses to soak in a warm
solution of soapy water will allow the blocks to come off the lens gently. I wonder if by this process, the hydrophobic coating may stay on the lens after "deblocking"

[OpticalTim]

We use the horizon 3 edger and also had a problem with marks being left on the lenses. Our solution was to use the blue lens savers, manually reduce the pressure to 40PSI and not try to take the lens down to size in 1 cut, especially a higher minus(over -3.00) in one of the smal eye sizes popular today. Attention also needs to be paid to the condition of the cutter blade. It's recommended the cutter be changed every 300 cuts but with a higher volume of polycarbonate lenses should be changed more frequently. Overkill? Listen to your edger, you'll HEAR the difference as it labors harder. The cost of a blade is much less than a spoiled progressive lens with AR!!

[Clive Noble]

I think you've all summed it up correctly, length of time of pad on lens and pressure of lens on edger, both blocking and cutting.

Having said that, I've never seen the problem with Crizal or with the better quality Stock Finished AR coated lenses.

Another problem I've found is after tinting a PAL, if I reapply the plastic verification sticker, on removal (after say a couple of hours) there is a lighter patch where the sticker has been and the dye has somehow bleached out.